Precise relationships between microvascular dimensions and the diffusion characteristics of gas exchange organs are not well understood. This is due, in large part, to difficulties in making direct, simultaneous measurements of gas exchange and vascular dimensions in the lung. Such measurements, however, can readily be made on a different respiratory organ, amphibian skin. This grant proposes to evaluate some basic principles of gas exchange by making direct measurements on amphibian skin to determine how the membrane diffusing capacity to carbon monoxide (DMCO) is affected by changes in capillary diameter, the density of perfused capillaries, the thickness of the blood-air diffusion barrier. A new method using all in vivo measurements will be performed to determine DMCO and the gas uptake capacity of erythrocytes while they flow through capillaries. These determinations will test the following hypotheses: 1) Changes solely in capillary diameter have no affect on DMCO, 2) Changes in the density of perfused capillaries have a larger effect on DMCO at low capillary density than at higher capillary densities. 3) As the thickness of the blood-air diffusion barrier increases, changes in the density of perfused capillaries have less effect on DMCO. The role of diffusing capacity changes in the regulation of 02 and C02 exchange in amphibian skin will also be evaluated. Finally, a mathematical model will be developed using known relationships of heat flux from pipes to describe diffusive gas flux across amphibian skin. Experiments will be performed on pithed anuran amphibians. The net fluxes of C0, C02 and 02 will be measured across a small patch of skin by mass spectrometry. DMCO and skin conductance to 02 and C02 will be determined. Simultaneous with these flux measurements, capillary diameter and capillary density in the same region of skin will be determined by video microscopy. Changes in capillary density will be elicited by topical application of phenylephrine to the skin. Capillary diameters will be altered using a pump-perfused skin preparation in which intravascular pressure can be modulated. The thickness of the blood-air diffusion barrier will be measured using standard histological techniques. Different amphibians with a variety of epidermal thickness will be tested and compared. This investigation of amphibian skin will provide unique information helpful to the understanding of basic issues in mammalian gas exchange that have not been experimentally accessible because of the complexity of the mammalian lung.